1. Field of the Invention
This invention relates in general to apparatus and circuits for monitoring the supply of electrically conductive writing fluid in a reservoir for ink printer devices.
2. Description of the Prior Art
In printer devices that use ink as the writing medium, the write head generally moves along a recording medium driven by a motor means and is supplied with writing fluid from an ink reservoir through a supply line. When the ink reservoir is integrated in the write head, then the reservoir moves with the write head and visual monitoring of the ink supply and the container is not possible. A constant monitoring of the ink supply, however, is necessary for a reliable functioning of the ink printer especially for printer devices in equipment such as telex and data traffic equipment.
U.S. Pat. No. 4,196,625 discloses an apparatus for monitoring the supply of electrically conductive writing fluid for ink printer devices. In this apparatus, the conductivity of the ink is utilized for measuring the supply. For this purpose, three measuring electrodes are introduced into two separate cavities or troughs of an ink reservoir which are separated by a web and the electrical resistance between the electrodes is measured using a resistance bridge. Two of the electrodes are mounted in the same cavity of the reservoir and serve for the acquisition of a fluid associated comparison resistance and two of the electrodes which are situated in different cavities serve for the acquisition of the resistance changing value which depends on the level of the fluid. So as to avoid decomposition of the ink by electrolysis, rectangular pulses of a pulse generator are supplied to a resistance bridge through a capacitor as zero symmetrical alternating voltage such that one branch or arm is composed of the ink resistances to be acquired and the other branch or arm is composed of fixed resistors and the output signal is forwarded to an evaluation circuit and to a circuit component having a threshold which produces an output signal for a display means when a comparison value corresponding to a defined supply volume goes below the defined volume.
The monitoring of the ink supply using an alternating current bridge circuit and subsequent mean value formation is relatively complex and an additional measurement of the "impending end of ink" increases the circuit outlay considerably.